Generally, refrigerant circuits for use in an air conditioner are constituted by a compressor, an outdoor heat exchanger, an expansion valve and an indoor heat exchanger which are all connected in series to perform operation on the vapor-compression refrigerating cycle. In such refrigerant circuits, operation is sometimes performed in inadequate conditions owing to acceleration/deceleration of the compressor, abrupt fluctuations in load, and defective operation of other apparatus than the compressor. If this happens, the compressor gets internal damage, resulting in a failure in the compressor.
To cope with this, the conventional refrigerant circuits of this type are designed to prevent liquid compression and the dilution of the lubricating oil, for example, by controlling the degree of superheating a suction refrigerant (hereinafter referred to as “the degree of suction superheat) in the suction pipe of the compressor.
The above refrigerant circuits control the temperature of the discharge pipe of the compressor so as to become equal to a temperature which is determined by making a comparison between the saturation temperature of the low-pressure refrigerant and the temperature of the discharge gas of the compressor, whereby degradation of oil inside the compressor as well as degradation of the magnets is avoided.
Concretely, Japanese Patent Publication Kokai No. 7-180933 discloses a refrigerator according to which, the temperature of the oil of the compressor and the pressure of the suction refrigerant of the compressor are detected and, according to the detection result, the solubility of the refrigerant dissolved in the oil is calculated. And, based on the solubility of the refrigerant, the operation frequency of the compressor is controlled, thereby preventing the dilution of the oil.
Also, Japanese Patent Publication Kokai No. 2001-99070 discloses a compressor having ultrasonic probes at the lubrication points thereof. Based on ultrasonic intensities detected by the ultrasonic probes, it is determined whether or not poor lubrication occurs.